Mercury-wetted fixed electrode electric arc generator



Dec. 6, 1966 P, K, LUDWlG ETAL 3,290,615

MERCURY-WETTED FIXED ELECTRODE ELECTRIC ARC GENERATOR Filed June 25, 1965 E@ -l ELE E i ww@ blk* United States Patent O 3,290,615 MERCURY-WETTED FIXED ELECTRODE ELECTRIC ARC GENERATOR Peter K. Ludwig, South Bend, Ind., and Juan T. DAlessio, Buenos Aires, Argentina, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed `lune 23, 1965, Ser. No. 466,489 Claims. (Cl. 331-127) This invention relates to apparatus for generati-ng an electric arc of very short duration, and more particularly to apparatus for generating electric arcs by means of discharging an electric current between mercury-wetter electrodes in a pressurized gas.

High speed electric arc generators may be used in a number of different applications. For instance, the light pulses accompanying the arc discharge may be used to test the characteristics of photosensitive devices. An advantage in generating the light pulses by means of an arc discharge is that the light pulses occur simultaneously with electrical pulses which may be used to indicate the time duration and relative intensity of the light pulses. The electrical pulse may also be used independently of the light pulse, for example, as a fast-rise-time pulse generator.

The idea of generating an electric are between mercurywetted contacts in a pressurized gas has been previously described in the literature. One such generator was described in The Review of Scientific Instruments, vol. 30, No. 1 (January 1959), Generator of Nanosecond Light Pulses for Phototube Testing, Ker-ns et al. Essentially, the generator described therein produces light pulses by applying a D.C. voltage to the contacts of a vibrating reed relay and inducing a series of arc discharges between the mercury-wetted contacts as the movable contact arm is driven toward the stationary contact in response to an external magnetic field which is excited by a sixty-cycle voltage.

In other words, when the distance between the two contacts becomes suiciently small relative to the voltage across the contacts, an arc discharge occurs and the pressurized hydrogen acts to quench the arc. Each arc serves to discharge the capacitor formed by the contacts and the voltage must recover to the discharge potential before another arc an occur. However, while the inter-contact capacitor is charging, the movable contact approaches the stationary contact lowering the discharge potential. Consequently, the D.C. voltage across the gap decays in a series of steps of decreasing size and a group of arc discharges is produced on each mechanical cycle of the movable contact arm. The purpose in having the movable contact repeatedly engage the stationary contact, as taught in the prior art, is to mainain a mercury lm on both contacts by transferring some of the mercury from the stationary contact which is partially immersed in mercury to the movable contact which is not immersed in mercury. See Harrisons U.S. Patent No. 2,302,546, dated Nov. 17, 1942, col. 3, line 75 through col. 4, line 7 and col. 5 lines 10 through 14. A

Mercury wetting of the contacts is essential to prevent pitting which would alter the eld intensity and aifect he discharge potential as well as cause erosion of the contacts.

Certain disadvantages, overcome by the present invention, are inherent in the operation of the above-described device. Persons skilled in the art will note that the repetition rate of the pulses is not constant within a given group of pulses, that is during one mechanical cycle of the movable arm. The reason for this is that the discharge potential varies as a function of the space between the contacts Patented Dec. 6, 1966 which is constantly changing between arcs. This change in discharge potential further results in a reduced amplitude in each successive voltage pulse within a group and a reduced intensity in each successive light pulse within a group since the intensity of the light pulses is approximately proportional to the square of the gap voltage for high voltages. The variation is discharge potential and decrease in distance between contacts also cause variation both in the time duration of pulses and in the length of arc acting as a light source. Although these variations may not be substantial with respect to successive pulses, they may become appreciable when considering the difference in duration and length between the first and last pulse within a group.

It is to be further noted that the device described by Kerns et al. does not include a way to independently set the repetition rate of the pulses except by varying the frequency of the magnetic field excitation voltage. Even then, however, the recurrence of groups of pulses is limited to the relatively low frequency response of the mechanical vibrator.

Since the duration of the electrical pulses is in the nanosecond and sub-nanosecond range, distribution of the pulses can be effective only by means of transmission lines; consequently the arc generator must be capable of matching or at least approximating the characteristics of a transmission line. It is desirable that the arc generator be incorporated into a transmission line without causing a discontinuity therein by having a metallic shell acting as the outer conductor of the transmission line and having the two contacts acting as the inner conductor of the transmission line, as described in the above reference. The transmission line characteristics, however, cannot be maintained constant with the movable arm type of operation since the inter-contact capacity varies as the distance between the contacts changes.

One important object of the present invention is to provide a generator of very short electrical arcs wherein the discharging potential is allowed to recover to a predetermined and constant value before a succeeding discharge occurs.

Another object of the present invention is to provide a generator of very short electrical arcs with each arc lasting for the same time as all other arcs once the electrical circuit parameters have been fixed.

It is a further object of the present invention to provide apparatus for producing electrical arcs of very short duration wherein the repetition rate of the arc discharges is dependent only upon non-time-varying electrical circuit parameters capable of being independently set. In other words, it is an object of the present invention to obtain true periodicity in a series of arcs.

It is another object of the present invention to provide apparatus for producing electrical arcs of very short duration wherein the maximum repetition rate of arc discharges is not limited by the mechanical motion of a vibrating relay contact but only by the recovery time constant of the associated electrical circuitry together with the electrical characteristics of the generator.

It is yet another object of the present invention to provide a system for the distribution of electrical pulses in the nanosecond and sub-nanosecond range wherein the pulse generator may be incorporated into the distribution system and matched to the impedance characteristics thereof.

It is still another object of the present invention to provide a device for generating light pulses of very short duration by discharging an electrical potential between mercury-wetted electrodes in a pressurized gas wherein the physical length of the arc acting as the light source is fixed.

Briefly, the objects of the present invention are accomplished by incorporating two electrodes into an air-tight capsule containing pressurized hydrogen and a pool of liquid mercury and orienting the capsule so that the mercury contacts only one electrode and cannot form a mechanical bridge between both. The electrodes are treated so that mercury will wet the arc-sustaining surfaces and they are fixed so that a short, permanent gap is formed between the arc-sustaining surfaces of the electrodes. We have discovered, contrary to the teachings of the prior art, that during operation, that is, when repeated discharges take place between the fixed electrodes, the arc-sustaining surfaces of the electrode which has not contacted the mercury becomes coated with mercury. In other words, both electrodes become mercury wetted and remain so during operation, despite the fact that only one is in physical contact with the mercury. Our discovery permits the elimination from the Kerns et al. device of the motion of one of the contacts and the associated means for obtaining periodic mechanical engagement of the contacts, and avoi-ds all the disadvantage resulting therefrom.

Other objects of the invention will become obvious to persons skilled in the art upon reading the following disclosure in connection with the accompanying drawings in which:

FIGURE 1 is a diagrammatic representation in section of the arc generator of the present invention incorporated in a transmission line.

FIGURE 2 is an enlarged detailed drawing of the electrodes of FIGURE 1.

FIGURE 3 is a drawing of an alternate embodiment of the invention.

Referring then to FIGURE 1, an oblong glass envelope is fixed with its axis in a vertical position and forms the capsule or container of the arc generator. An electrode 12 is embedded at the top of glass envelope 10 and is fixed coaxially therewith. The electrode 12 has a pointed surface 16 formed radially from its axis. A second electrode 14 which is flat is attached to the glass envelope 10 at its bottom. The electrode 14 enters the bottom of the glass envelope 10 coaxially and is formed to define a fixed gap with the arc-sustaining, pointed surface 16 of the electrode 12. A pool of mercury 18 is situated at the bottom of the capsule 10 and surrounds the electrode 14. The mercury 18 is attracted by capillary action to wet the surface of electrode 14, as is known in the art.

The capsule 10 contains pressurized hydrogen indicated at 20, which is introduced into the capsule 10 through a Kovar tube 22 sealed to a side arm 21 of the glass capsule 10 with a 7052 glass bead. The Kovar tube 22 -is closed after filling the capsule 10 to the desired pressure by squeezing and plugging the open end 23 with silver solder.

A cylindrical metal shield 36 surrounds the glass capsule 10 with the electrodes 12 and 14 lying approximately along the axis of the cylindrical shield 36. The capsule 10 is securely mounted within the shield 36 by filling the intervening space with a nonuorescent insulating plastic 34 which acts as the dielectric material of a transmission line. The shield 36 acts as the outer conductor of a transmission line. The upper electrode 12 is attached to a conducting wire 24 which conductively couples a resistor 26 to electrode 12. The other terminal of resistor 26 is connected to a center conductor 30 of a feed-in transmission fline from a voltage source 28. The outer conductor 32 of the feed-in transmission line is connected to the grounded metallic shield 36 of the arc generator.

The electrical signal output from the generator is taken from the lower conductor 14 into a center conductor 33 of the output transmission 'line and fed to a load represented by referenced numeral 35. The outer conductor 37 of the output transmission line is likewise connected to the shield 36 of the arc generator.

FIGURE 2 shows the composition of the electrodes in more detail. Referring to FIGURE 2, the upperelectrode 12 is composed of a nickel feed-in wire 40 which is spotwelded to a tungsten wire 42 to allow for a pressure-tight glass to metal seal. The arc-sustaining surface 16 is nickel and spot-welded to the tungsten wire 42. Similarly, theV lower electrode 14 has a nickel feed-in wire 44 spot-welded to a tungsten feed-in wire 46 which is sealed to the glass capsule 10. The flat arc-sustaining surface 45 of the lower electrode 14 is spot-welded to the FIGURE 3 shows an alternate embodiment of the arc generator in which the side arm 21 is eliminated. A Kovar tube 50 acts as the feed through conductor for the upper electrode and has arc-Sustaining surface 52. Similarly, a Kovar tube 54 forms the lower electrode with an arc-sustaining surface 56. This allows for flushing as well as filling of the capsule.

Referring again to FIGURE 1, the operation of the device is as follows: The voltage source 28 has a value sufficient to cause an arc discharge across the gap formed by the pointed surface 16 of electrode 12 and the fiat surface of electrode 14. The pressurized hydrogen 20 acts to suppress the arc in an extremely short time. In effect, the capacitance formed by the two electrodes has been discharged and the voltage between the electrodes must charge this inter-electrode capacitance to a value sufficient to cause a second discharge. This recovery time is determined by the value of resistor 26, the value of the voltage source 28, the capacitance formed by the electrodes and any stray capacitance together vwith any external capacitance that might be added. Once the voltage recovers, however, the device operates repeatedly and arcs occur periodically. It is noted that the recovery time may be varied by adding an external variable capacitor across the electrodes and varying the recovery time, and hence the period, of the generator by varying the capacitor setting. Other ways to vary the period of the arcs include varying the value of resistor 26 (as shown in FIGURE 1) or the magnitude of the voltage source 28. We have observed repetition rates up to 50,000 pulses per second and operation over a lO-to-l range of voltage source magnitude.

The useful light pulses may be observed through a window (not shown) extending through the outer shield 36 and the plastic material 34 into the glass envelope 10. When used as a light pulse generator, the glass envelope 10 should be of a material which is transparent to the frequency spectrum of the light sought to be used.

As mentioned before, the improved characteristics of our device have resulted from our discovery that even though only one electrode contacts the mercury, repeated discharge between the electrodes will wet the arc sustaining surface of the electrode which does not contact the mercury. We do not suggest a theory for this phenomenon, but merely report that we have observed successful operation thereof. This teaching is in contrast with the accepted method of maintaining mercury wetting, as taught in the above-referenced Harrison patent, namely, repeated engagement of a movable contact with a stationary contact causing some of the mercury to be transferred to the contact-making area of the movable contact where it deposits and maintains a similar film or coating of the liquid. Our discovery, then, enables an arc generator of the type herein described to have the following advantages:

(1) The characteristics of the resulting light and electrical pulses are constant with respect to pulse width, electrical pulse amplitude and light pulse intensity, and periodicity;

(2) The device operates without an external coil to generate a magnetic field to drive a movable contact;

(3) The maximum repetition rate of arc discharges is increased by several orders of magnitude since operation is not dependent upon the mechanical motion of a vibrating reed;

(4) The repetition rate of arc discharges may be varied by adding a variable capacitance across the gap or a variable resistor in series with voltage source, or by changing the magnitude of the voltage source;

(5) The shape and spacing of the electrodes may be designed to minimize inter-electrode capacitance;

(6) 'Dhe device may be incorporated into an electrical pulse distribution system and matched to the impedance characteristics of a transmission line with a known and fixed inter-electrode capacitance;

(7) The arc length of the light pulse is fixed and constant; and

(8) Electrical pulse amplitude and light pulse intensity are insensitive to large variations in the magnitude of the voltage source.

Persons skilled in the art will readily adapt the teachings of the present invention to |uses far different from those illustrated. Accordingly, the scope of protection afforded the invention should be determined only in accordance with the appended claims and not limited to the uses above described.

The embodiments of the invention in which an exc1usive property or privilege is claimed are defined as follows:

1. Apparatus for generating an electrical discharge of very short duration comprising:

( 1) a gas-tight capsule containing a pressurized gas;

(2) first and second conducting electrodes extending into said capsule;

(3) means for continuously wetting the surface of one of said electrodes with a thin coat of liquid metal;

(4) means for fixedly mounting said first and second electrodes within said capsule to form a narrow spark gap defined on one side by a small surface area `of one of said electrodes; and

(5) means for applying a potential diiference between said electrodes sufficient to generate an electrical discharge across said spark gap causing the gap potential difference to decrease, whereby said pressurized gas quenches the spark.

2. Apparatus for repeatedly genertaing electric arcs of very short duration comprising:

(1) a gas-tight capsule containing a pressurized gas;

(2) a pool of liquid metal within said capsule;

(3) a first rigid, liquid-metal-wettable, conducting electrode extending into said capsule and adapted to contact said pool of liquid metal;

(4) a second rigid, liquid-metal-wettable, conducting electrode extending into said capsule and electrically isolated from said first electrode;

(5) means for forming a gas-tight seal between said capsule and said irst and second dielectrodes and for firmly attaching said electrodes to said capsule to form a fixed narrow gap within said capsule dened -by a small surface area of each of said electrodes; and

(6) means for applying a potential difference between said electrodes suflicient to generate an electric arc across said iixed gap whereby the gap potential difference decreases iand said pressurized gas quenches said arc, said arcs occurring periodically.

3. The apparatus of claim 2 wherein said capsule is cylindrical in shape and is surrounded by a coaxial metal shield forming the outer conductor of a transmission line, said first and second fixed electrodes forming the inner conductor of said transmission line.

4. The apparatus of claim 2 including variable means connected in circuit with said electrodes for varying the recovery time of the circuit, whereby the period of occurrence of said arcs is determined by the setting of said variable means.

5. The apparatus of claim 2 wherein the arc-sustaining area of one of said electrodes is substantially ilat and the arc-sustaining area of the other electrode is pointed, said pointed arc-sustaining area being disposed normal to the flat arc-sustaining area and having geometrical symmetry therewith.- i y References Cited by the Examiner UNITED STATES PATENTS 4/1939 Suits 313-182 X References Cited by the Applicant 2,264,022 11/ 1941 Ellwood. 2,289,830 7/1942 Ellwood. 2,302,546 11/ 1942 Harrison.

OTHER REFERENCES NATHAN KAUFMAN, Primary Examiner.

S. H. GRIMM, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,290,615 December 6, 1966 Peter K. Ludwig et allc It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 14, for "wetter" read wetted line 54, for "mainain" read maintain line 63, for "he" read the column 2, line 7, for "is" read in column 4, line 10, after "the", second occurrence, insert tungsten feed-in wire 46. column 5, line 41, for "genertaing" read generating column 6, line 5, for "dielectrodes" read electrodes Signed and sealed this 19th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer Commissioner of Patents EDWARD J. BRENNER 

1. APPARATUS FOR GENERATING AN ELECTRICAL DISCHARGE OF VERY SHORT DURATION COMPRISING: (1) A GAS-TIGHT CAPSULE CONTAINING A PRESSURIZED GAS; (2) FIRST AND SECOND CONDUCTING ELECTRODES EXTENDING INTO SAID CAPSULE; (3) MEANS FOR CONTINUOSULY WETTING THE SURFACE OF ONE OF SAID ELECTRODES WITH A THIN COAT OF LIQUID METAL; (4) MEANS FOR FIXEDLY MOUNTING SAID FIRST AND SECOND ELECTRODES WITHIN SAID CAPSULE TO FORM A NARROW SPARK GAP DEFINED ON ONE SIDE BY A SMALL SURFACE AREA OF ONE OF SAID ELECTRODES; AND (5) MEANS FOR APPLYING A POTENTIAL DIFFERENCE BETWEEN SAID ELECTRODES SUFFICIENT TO GENERATE AN ELECTRICAL DISCHARGE ACROSS SAID SPARK GAP CAUSING THE GAP POTENTIAL DIFFERENCE TO DECREASE, WHEREBY SAID PRESSURIZED GAS QUENCHES THE SPARK. 